1. Field of the Invention
This invention relates to heterojunction bipolar transistors (HBTs) with a graded superlattice between the base and emitter.
2. Description of the Related Art
HBTs represent a field of increasing interest because of their potential for higher emitter efficiency, decreased base resistance, less emitter current crowding, improved frequency response and wider temperature range of operation. A representative HBT is illustrated in FIG. 1. It includes a semi-insulating semiconductor substrate 2, typically InP or GaAs. A highly doped n.sup.+ subcollector layer 4 on the substrate provides an underlying contact with the more lightly n.sup.- doped collector layer 6, with a metallized or highly doped semiconductor pad 8 providing a collector contact through the subcollector. A p.sup.+ base layer 10 is formed over the collector, with an n doped emitter 12 over the central portion of the base and a lateral base contact 14 surrounding the emitter. An emitter contact 16 is provided on the emitter's upper surface.
During the operation of the HBT, the input voltage forward-biases the base-emitter junction, allowing electrons from the n-type emitter to enter the base. The injected electrons travel across the base by diffusion or drift, and are swept into the collector when they reach the base-collector junction by the high fields in this region. In a homojunction bipolar transistor there is a flow of holes from the base into the emitter across the forward biased base-emitter junction. To suppress the reverse hole injection, the base doping is made lower than that of the emitter. This, however, makes the base layer more resistive, thereby increasing the device's overall base resistance and degrading its bandwidth. In an HBT, by contrast, a wide-bandgap emitter material is used that creates a higher energy barrier for hole injection into the emitter than the barrier to electron flow into the base, thereby automatically suppressing the base current that is due to hole injection. The doping of the base layer can be made as large as possible from solid state chemistry considerations, and the base resistance is correspondingly reduced.
To achieve the high base dopings necessary for RF performance, typically 5-10.times.10.sup.19 cm.sup.-3, beryllium is widely used as the p-type base dopant for npn HBTs. A significant reliability issue with such devices is that the electric currents and fields generated at the base-emitter junction cause Be to diffuse from the base into the emitter. This movement of Be is characterized by an increase in the emitter-base turn-on voltage as the electrical junction (defined as the location where the Be doping from the base meets the emitter doping) is pushed further into the emitter. The Be penetration leads to decreased gain, and eventually to complete failure of the transistor. This degradation mechanism is currently the operating rate limiting failure mechanism of npn AlInAs/GaInAs HBTs.
One approach to solving the Be diffusion problem uses an undoped layer of low bandgap material, such as GaInAs or GaAs about 100-500 Angstroms thick, between the wide bandgap emitter and the low bandgap base. The undoped layer acts as a buffer region into which the base Be can diffuse, so that the bulk of the diffusing Be does not reach the emitter. This technique is described in Malik et al., "High-gain, high frequency AlGaAs/GaAs graded band-gap base bipolar transistors with a be diffusion set back layer in the base", Applied Physics Letters, Vol. 46, No. 6, pages 600-602 (1985), and in Jalali et al., "New ideal lateral scaling in abrupt AlInAs.InGaAs heterostructure bipolar transistors prepared by molecular beam epitaxy", Applied Physics Letters, Vol. 54, No. 23, pages 2333-2335 (1989). Undoped setback or spacer layers of base material between the base and emitter are also disclosed in Jensen et al., "AlInAs/GaInAs HBT Ic Technology", IEEE Journal of Solid-State Circuits, Vol. 26, No. 3, March 1991, pages 415-421, and in Nottenburg et al., "InP-Based Heterostructure Bipolar Transistor", Proc. of the 1989 GaAs IC Symposium, 1989 pages 135-138. The setbacks, however, simply increase the distance that Be must diffuse before it penetrates into the emitter, and do not act to actually inhibit the Be movement. Furthermore, the electron transit time through the material increases with the width of the setback layer, and this in turn slows the speed of the HBT.
As the present invention employs a form of graded superlattice between the base and emitter to block Be diffusion, it would be helpful to briefly review the graded superlattice structure. Graded superlattices have previously been used to eliminate the interface pile-up effect of holes in a "high-low" InP/GaInAs avalanche photodiode (Capasso et al., "Pseudo-quaternary GaInAsP semiconductors: A new Ga.sub.0.47 In.sub.0.53 As/InP graded gap superlattice and its applications to avalanche photodiodes", Applied Physics Letters, Vol. 45, No. 11, Dec. 1, 1984, pages 1193-1195), and to minimize the effect of carrier trapping at the AlInAs/GaInAs interface of a metal-semiconductor-metal photodiode (Wada et al., "Very high speed GaInAs metal-semiconductor-metal photodiode incorporating an AlInAs/GaInAs graded superlattice", Applied Physics Letters, Vol. 54, No. 1, 2 January, 1989, pages 16-17). They consist of a series of layer pairs interfacing between first and second material, with each pair having one layer of the first material and another layer of the second material. Although the total pair thickness generally remain uniform, the thickness of each of the two layers varies from pair to pair. At the end of the superlattice adjacent the first material, the thickness of the layer formed from that material is at a maximum, while the thickness of the second material layer is minimum. These thicknesses are graded across the superlattice, so that at its opposite end the thickness of the first material layer is at a minimum and the thickness of the second material layer is at a maximum.
Graded superlattices have not previously been used in an HBT to block the diffusion of a base dopant. In Won et al., "Self-Aligned In.sub.0.52 Al.sub.0.48 As/In.sub.0.53 Ga.sub.0.47 As Heterojunction Bipolar Transistors with Graded Interface on Semi-Insulating InP Grown by Molecular Beam Epitaxy", IEEE Electron Device Letters, Vol. 10, No. 3, March 1989, pages 138-140, a graded interface is provided between the base and emitter that makes a linear and continuous transition from the base to the emitter material. This is accomplished in a molecular beam epitaxy (MBE) system by progressively increasing the temperature of the emitter material cell and reducing the temperature of the base material cell as the epitaxial growth from the base to the emitter proceeds. With an InGaAs base and InAlAs emitter, compositional grading of a base-emitter interface was achieved by simultaneously raising the Al and lowering the Ga cell temperatures, while keeping the total Group III flux constant. Graded interfaces of this type have been used to avoid conduction spikes at the base-emitter junction, and thus enhance charge transport and current gain, but do not solve the Be diffusion problem. Furthermore, it is difficult to cool the cell for the base material fast enough for a typical MBE growth rate of 100 Angstroms per minute; with typical cell temperatures of about 1,040.degree. C. for Al and 880.degree. C. for Ga, it would be necessary to reduce the cell temperature by about 100.degree. C. to reduce the flux from that cell by 90%.
In Asbeck et al., "InP-based Heterojunction Bipolar Transistors: Performance Status and Circuit Applications", Second Int'l. Conf. on Indium Phosphide and Related Materials, Apr. 23-25, 1990, pages 2-5, a short period superlattice was formed between an AlInAs emitter and a GaInAs base. The superlattice consisted of four sections, with each section having four identical periods; each period had a thickness of 12 Angstroms. In the first section adjacent the emitter, the four periods each had AlInAs and GaInAs layers with widths in the ratio 4:1; the width ratio was reduced to about 3:2 for each of the identical periods in the second section, to about 2:3 for the identical periods in the third section, and to about 1:4 for the identical periods in the fourth section closest to the base. This type of superlattice was proposed as an alternative to a continuous grading, because of problems in the precise control over the continuous grading process. There was no claim to any reduction in the field-enhanced diffusion of Be from the base, and in fact the present invention has found the Asbeck et al. superlattice structure to not be oriented towards a reduction of Be diffusion. The very narrow widths of the minority material layers in the sections at each end of the superlattice, on the order of 1.25 Angstroms, have not been found to be effective in reducing Be diffusion. Since a reduction in Be diffusion is a well known and important goal for an HBT with this type of material system, the lack of any reference to such a reduction indicates that no significant reduction in Be diffusion actually occurred.